1. Technical Field
The present invention relates to a magnetic recording medium including a back coat layer.
2. Background Art
As recordings are made at shorter wavelength and higher density than ever before, the magnetic layer of a magnetic recording medium is required to have higher surface smoothness and more improved running stability. Thus, the importance of the back coat layer used with the magnetic recording medium as means for achieving improved surface smoothness and running stability becomes now steadily increasing.
The electromagnetic characteristics of the magnetic recording medium are enhanced by improving the surface roughness (Ra) of the back coat layer and thereby making the surface thereof smooth. However, the magnetic recording medium drops in running stability, as the surface of the back coat layer becomes smooth or the coefficient of friction .mu. of the back coat layer increases. As the surface roughness of the back coat layer is increased to enhance the running stability of the magnetic recording medium, there is a drop of the surface properties of the magnetic layer, resulting in a drop of the electromagnetic characteristics thereof. For the back coat layer, it is thus desired that its surface be made so smooth that its surface properties can be improved with a drop of the coefficient of friction .mu.. This enables the magnetic recording medium to be improved in terms of running stability and yet without giving rise to any considerable deterioration of the electromagnetic characteristics of the magnetic layer.
One way to achieve smoothness is to decrease the average particle diameter of carbon black, fine inorganic particle or other filler contained in the back coat layer. This enables the surface of the back coat layer to be smoothened with some improvements in Ra and electromagnetic characteristics, but causes the coefficient of friction .mu. to become too high to obtain good-enough running stability.
To solve these problems, it has been put forward to use two kinds of filler (e.g., carbon black or fine inorganic) particles of varying particle diameters, use fillers and binders at specific quantitative ratios, or place some specific limitation on the average surface roughness Ra, as typically set forth in JP-A 54-21248, JP-A 57-53825, JP-A 59-14127, JP-A 60-45938, JP-B 4-79054, JP-B 4-81256, and JP-B 4-81261.
All these proposals make use of fillers having extremely varying particle diameters for the purpose of lowering the average surface roughness Ra, thereby achieving good-enough electromagnetic characteristics and decreasing the coefficient of friction .mu.. However, the combined use of these two kinds of carbon blacks or other fillers, if they were less dispersible in the back coat layer, would often give rise to an increase in Ra, resulting in some deterioration of the surface properties of tape.